The present invention relates to an inverter apparatus used in an air conditioner or the like.
A conventional inverter apparatus, as shown in FIG. 26, is configured in a manner that a current from a direct current source 5 is converted into an alternating current by an inverter apparatus 1, so that a motor 7 is driven by the alternating current. On the other hand, the inverter apparatus 1 comprises a function of power conversion. Therefore, a load is not limited to the motor 7, and a resistance load (a resistor) 6 such as an electric heater can be used as shown in FIG. 28 by adjusting a frequency of the alternating current and an output voltage. In an air conditioner for a vehicle, an electric compressor is driven by a built-in motor in air cooling operation, or the electric heater is electrified in air heating operation. Effective use of the inverter apparatus by the above-mentioned method is disclosed in the Japanese published unexamined patent application Hei 7-304325. In this case, as shown in FIG. 30, the motor 7 and the electric heater (resistors 6) are alternatively switched as the load of inverter apparatus 1.
In the configuration of the above-mentioned conventional inverter apparatus, a voltage of the direct current source 5 is pulse-width-modulated at about 10 KHz of carrier frequency in alternating current conversion. Therefore, in the case of the resistance load 6, a current having the same waveform as that of the voltage passes through the resistance load 6. As a result, as shown in FIG. 29, a rectangular wave current including a high frequency harmonic current having a high energy is fed to the resistance load 6, and electric noise is likely to be emitted outward. In other words, the inverter apparatus becomes an electric noise source. Furthermore, the below-mentioned problem occurs in the case that a sealed compressor which includes a motor therein and is mounted on an electric automobile and is driven by the inverter apparatus. In the sealed compressor including the motor, refrigerant and lubrication oil enclosed in the compressor works as dielectric substance, and hence a large capacitance is formed between a motor coil and the enclosure of the sealed compressor. In operation at a high carrier frequency such as 10-20 KHz, a part of the high frequency harmonic current is liable to leak outward through the enclosure of the compressor. In this case, the inverter apparatus becomes an inefficient current source which generates large electric noise. The large leakage current gives a harmful influence to electronic circuits of the automobile, and furthermore generates electromagnetic wave noise which exerts a harmful influence on the environment.
Since the pulse-width modulation is performed by switching operation, a peak voltage applied to the resistance load 6 is the voltage of the direct current source 5. Therefore, a withstand voltage of the resistance load 6 must be selected to have a voltage higher than that of the direct current source 5. The resistance load having a high withstand voltage is large in size and expensive in cost. A maximum voltage of the power source of an electric car reaches 450 volts, and it is a problem to cope with a high withstand voltage.
Furthermore, as shown in FIG. 27, waveforms i.sub.u, i.sub.v, i.sub.w of three-phase output currents are relevant to each other, and are not independent. Therefore, it is difficult to adjust individually each power of three resistance loads 6 which are configured in delta-connection (or Y-connection). In order to adjust the power individually, three inverter apparatus are required. On the other hand, in the case of one resistance load 6, the resistance load 6 is connected across two phases of three phases, therefore, one phase is not used and a burden is imposed on only two phases. Consequently, it is difficult to utilize equally the respective three-phase outputs without deflection and extract the maximum electric power.
In the case where the load of the inverter apparatus 1 comprises the motor 7 and the resistance load 6 (electric heater or the like) as shown in FIG. 30, and these loads are alternatively used by switching frequently with a switch 30, it is desirable that generation of arc or surge is reduced, so that durability is improved and the switch 30 is reduced in size.
The present invention is to solve the above-mentioned outstanding problems, and proposes to provide an inverter apparatus which is easily applied to a load other than the motor.